TechArchive

Original article date: December 1998

HSE turns the screw still further

A new substitute material for the high density asbestos-cement type “Sindanyo” boards used for electrical and thermal insulation applications in general industry has been announced by Cape Insulation.

Asbestos-cement products have been used for years in the electrical industry for the construction of mechanically strong and heat-resistant rods tubes cylinders and plates. Applications include panel boards arcing barriers and insulating tubes and cylinders used on the construction of air-cooled transformers. Sindanyo is a proprietary high density cement-based board used for a variety of mechanical thermal and electrical applications in general engineering particularly at elevated temperatures. It was manufactured from asbestos fibres and cement formed into fully compressed boards under high pressure. It has good heat and arc-resisting properties and good anti-tracking properties.

Non-asbestos grades of Sindanyo were produced but the mechanical properties do not match the original either at ambient or elevated temperatures though they have been sufficient to satisfy some applications. Other substitutes have been based on ceramic fibres and resins. For arc chutes and boxes compression moulded glass-reinforced polyester (DMC) is satisfactory for millisecond exposure to temperatures up to 1600degC. Mica sheets sometimes impregnated with silicone resins have also been investigated.

But this has left quite a large number of applications which have been difficult to address with substitute materials. The announcement of a new substitute is timely since it comes on the back of the publication by the Health and Safety Executive (HSC) of a consultative document setting out regulatory proposals to restrict yet further the importation supply and use of white asbestos (chrysotile).

Cape’s Industrial Sales Manager David Lemmings acknowledges that the new material called Duraflex isn’t yet fully proven since the company is still working on amendments and processing refinements to give optimum properties but it is available in commercial quantities. The material has been beta-tested at remarkable speed since it is only four months since the first sample materials went into service. Typical targeted applications are the top plates and clout plates in induction furnaces arc chutes used for electrical earth shielding and deal plates and pushers in the glass industry. However machinable insulation is also used in a whole host of miscellaneous industrial applications. Cost indications suggest that the material is 10-12% cheaper than the asbestos alternatives and the expected service life is longer. In effect therefore the material may be as much as 20% cheaper.

Duraflex is based on a reinforcement of cellulose fibres and mica in a calcium silicate matrix but does exhibit toughness and flexibility not seen in traditional asbestos replacement materials. It is non-wetting to molten metals and exhibits excellent outgassing characteristics when used under vacuum. Some comparative properties are listed in Table 1 compared to the conventional calcium silicate material Calsil and a typical asbestos-cement material of the Sindanyo type.

Cape is cautioning that Duraflex is still a developmental product and there are still changes in formulation being investigated. The material must be dry to achieve its peak performance though this limitation may be overcome in the future the company says. Properties are also strongly directional though variations in formulation can suppress the anisotropy ton some degree at the expense of peak property values.

* difficult to measure nowadays ** ultimate crushing strength *** continuous heat soak

• Cape Insulation Products
• 0191 416 1111
• David Lemmings

December 1998